1. Field of the Invention
The present invention relates to the petrophysical field and more particularly to the determination of brine saturation within a porous medium such as a geological formation.
2. Description of the Prior Art
Knowledge of the brine saturation allows calculation of the amount of hydrocarbons in place in the case of an oil reservoir, or the amount of CO2 stored in the case of CO2 storage. It is therefore very important for the specialist in charge of the development of an underground formation, within the context of hydrocarbon production or acid gas storage for example, to know the brine saturation of the formation.
It is well known to determine the saturation of a geological formation by carrying out electrical measurements within wells, while drilling or not. These measurements in a well are referred to as well logs. These electrical logs measure the electrical resistivity of the medium. They have to be analyzed in order to deduce the saturation values of the formation.
A conventional method for carrying out such a resistivity log interpretation uses Archie's laws, and notably the following relation:
  RI  =                    R        t                    R        0              =          S      w              -        n            with:
RI: resistivity index
Sw: brine saturation of the medium
Rt: electrical resistivity of the medium
R0: electrical resistivity of the 100° brine-saturated medium (Sw=1)
n: an integer.
The electrical resistivity of the medium Rt is measured by electrical logging, whereas the electrical resistivity of the 100% brine-saturated medium R0 is determined from measurements performed on cores.
An important stage in the interpretation of resistivity logs (Rt) in terms of brine saturation (Sw) is defining parameter n of the above relationship.
It is therefore well known to measure, on the one hand, the resistivity index and the brine saturation on samples taken from the geological formation. These measurements are often carried out under the pressure and temperature conditions encountered within the formation. Thus, by measuring several pairs (RI, Sw), parameter n of Archie's relation can be estimated.
Measurement of the resistivity index of a porous medium is the most difficult experimental stage in this interpretation of the electrical response of porous media.
A possible technique for measuring the resistivity index and the brine saturation on a large number of samples places these samples on a semi-water permeable porous plate arranged in an enclosure that can be pressurized. When pressure P in the enclosure is progressively increased, the saturation of the samples decreases progressively. At each pressure stage, the samples are removed from the enclosure and the saturation is determined by weighing. The resistivity is measured by applying two metallic plates connected to a measuring device onto the two faces of the sample.
However, it is known that saturation determination by means of this technique is destroyed by errors due to a loss of grains of the sample during the experiment and that the resistivity measurement is destroyed by errors due to the contact resistances between the plates and the porous medium.
Furthermore, for low-permeability samples, a porous plate of very low permeability is necessary, requiring long experimental times (of the order of 2 months at least).